1. Field of the Invention
The present invention relates to an optical multilayer thin film, and a beam splitter formed with the thin film and suitable for use in various optical apparatuses such as optical measuring instruments, office automation apparatuses, laser application apparatuses, electron image processing apparatuses, optical communication apparatuses, optical information processing apparatuses, optical manufacturing apparatuses, etc.
2. Description of the Prior Art
In laser application apparatuses such as a laser beam printer, optical disc apparatus, laser processing apparatus, measuring instrument, etc., beam splitters are widely used to split a single optical beam into two paths. The beam splitter is formed by interposing an optical multilayer film between two sloping surfaces of two prisms. The optical multilayer film is formed by laminating a plurality of thin films of inorganic substances such as TiO.sub.2, SiO.sub.2, and MgF.sub.2 on the surface of an optical part such as a prism or the like in accordance with a vacuum deposition technique, for instance.
One of the important performance requirments for the beam splitter described above is that there is no difference in phase between S and P polarized components of reflected and transmitted light in the beam splitter (hereinafter, referred to as "no phase difference"). In other words, linear polarized light can be obtained. Here, S is an abbreviation of "senkrecht (German, meaning perpendicular)", and an S polarized component means a polarized component of reflected and transmitted light having vibrations perpendicular to the plane of incidence of the light. Further, P is an abbreviation of "parallel (German, meaning parallel)", and a P polarized component means a polarized component of reflected and transmitted light having vibrations parallel to the plane of incidence of the light. In other words, an S polarized component means a component of light which is perpendicularly polarized with respect to the plane of incidence, and a P polarized component means a component of light which is polarized in a direction parallel with respect to the plane of incidence.
The reason why no phase difference is required is as follows. Namely, if there is a difference in phase between the S and P polarized components of reflected light, the spot diameter of a laser beam cannot be minimized, thus resulting in a problem in that the S/N (i.e., signal-to-noise) ratio is unavoidably reduced.
A beam splitter having no phase difference can be realized or achieved dependent upon the design of the optical multilayer film to be interposed between the two prisms. In the conventional optical multilayer thin films having no phase difference, there is a tendency in characteristics such that the S polarized component of reflected light is significantly larger than the P polarized component of the same reflected light.
Accordingly, it has been impossible for the conventional beam splitter, which uses the optical multilayer thin film to satisfy a requirement that the S and P polarized components of reflected light must be substantially equal to each other. Such a requirement exists, for example, when an optical path is designed so as to be used in common for both an MO (magnetooptic) element and a servo element in an photomagnetic disk reading apparatus. Therefore, when an optical system including such a beam splitter is designed, there is a problem in that the positional relationship among the elements such as the MO element and the servo element or the like arranged in the system is restricted from the design standpoint, so that the design flexibility is reduced. In addition, in the conventional beam splitter, since the reflectances of the S and P polarized components of reflected light is dependent upon the wavelength, it has been impossible to make constant the reflectances of the two components in a wide wavelength range.
Further, in designing an optical system, there is a case where the reflectance and transmittance of a beam splitter are required to be modified. With respect to this requirement, however, the optical multilayer film designed so as to have no phase difference is provided with an inherent reflectance (or transmittance). Therefore, when the reflectance is required to be modified without deteriorating the no phase difference performance, the optical multilayer film itself must be designed in a different way, because it is insufficient to change only the prism characteristics.
Further, in the optical system design, it is possible to increase the design flexibility, if the angle of incidence of a light beam upon the beam splitter can be modified. In the prior art beam splitter, however, there is a problem that once the angle of incidence of a light beam upon the beam splitter is changed, it has been impossible to maintain the no phase difference conditions due to the characteristics of the optical multilayer film.
As described above, in the prior art beam splitter, it has been difficult to change the various conditions such as the fractions of the reflectance of the S and P polarized components of reflected light, the refractive index, the angle of incidence, etc., while maintaining the no phase difference conditions, thus raising a problem in that the flexibility of optical system design is extremely restricted.
On the other hand, another performance requirement of the beam splitter as described above is to obtain a high resolving power for both the S and P polarized components of reflected light. In a beam splitter satisfying the above-mentioned requirements, there is provided a multilayer polarizing light separating film which is one of optical multilayer films.
For instance, such a multilayer polarizing light separating film is formed on the polarizing beam splitter used in the photomagnetic disk. In the prior art polarizing beam splitter, however, only the S polarized component of the reflected light reflected from an optical disk is separatable.
Accordingly, there is a problem in that the various optical elements for composing an optical system (i.e. an optical disk, polarized light beam splitter, separated polarized component detecting device, etc.) are unavoidably arranged at fixed positions, thereby reducing the design flexibility of the optical system.